1. Field of the Invention
The invention relates generally to communications networks and more particularly to communications networks that operate on a periodically executed schedule.
2: Discussion of the Related Art
FOUNDATION™ FIELDBUS technology is an open technology provided by the FIELDBUS Foundation of Austin, Tex., U.S.A. It is a local area network type of communications BUS based on the International Electro-Technical Commission 61158 FIELDBUS standard. That standard calls for some of the periodic communications on the BUS to be scheduled within a periodically executed schedule.
While Foundation FIELDBUS technology permits interactive communication among devices, scheduling is restricted to some degree. For instance, only one function block in a single application processor device may execute at a time. Further, only one device may publish (that is, a publication or back-calculation publication) on the link at a time, and client-server activity may only take place between publications.
Function blocks are typically scheduled in “natural order”, from input to output, following primary connections. Particularly when many devices are sharing a link, “natural order” scheduling tends to produce a large number of relatively “short” publication gaps. These “short” gaps often reflect wasted bandwidth because large queued communications, and all communications queued behind them, are delayed until a large enough gap arises in the schedule. For example, display call-ups and operator station display call-ups are often most severely affected since they typically require relatively large gaps to satisfy their heavy information transfer demands. Accordingly, the number of client-server messages transmitted per macrocycle is limited, and less than optimum application performance is achieved.
“Natural order” scheduling also often yields relatively long schedules and frequently produces significant loop latencies. Since large latencies reflect older samplings and ineffectual performance, their appearance in critical control applications is often a matter of great concern.
Thus, an improved method for scheduling is needed. Such a method should allow for greater communications usage from a fixed bandwidth; shorten the overall scheduled portion of the link schedule; and, minimize loop latencies to achieve better control.